Mad cow disease was first reported in the United Kingdom in 1986, peaking in 1993 with almost 1,000 new cases per week. In 1996, variant Creutzfeldt-Jakob Disease (vCJD) was detected in humans and linked to the mad cow epidemic. Eating contaminated meat and cattle products is presumed to be the cause.Both are fatal brain diseases with unusually long incubation periods, often lasting years.Source: CDC

WASHINGTON (CNN) --The first apparent case of mad cow disease in the United States has been discovered, the U.S. Department of Agriculture said Tuesday.

Two tests have been carried out on meat from the cow, enabling Agriculture Secretary Ann Veneman to call the case a "presumptive positive." A sample is being flown to England for a third test to confirm the case.

Bovine spongiform encephalopathy, or BSE, is linked to a similar form of the incurable and fatal brain-wasting disease in humans, called variant Creutzfeldt-Jakob Disease, or vCJD. There have been a small number of cases of vCJD reported worldwide, primarily in the United Kingdom, in people who ate BSE-contaminated meat.

Within hours of the announcement, an official with Japan's agriculture ministry told CNN that his country would ban imports of U.S. beef. South Korea, Taiwan, Malaysia and Singapore all followed suit Wednesday.

U.S. officials believe that meat from the animal left a processing plant, Veneman said, and an investigation is under way to determine if any has reached store shelves.

However, that does not mean the public is in danger, Veneman said.

"One important thing to remember is that muscle cuts of meat have almost no risk," Veneman said, emphasizing that the disease is typically spread by consumption of brain or nerve tissue, which did not enter the food system. "I know of no science to show that you can transmit BSE from muscle cuts of meat."

The sick animal came from a farm in Mabton, Washington, about 40 miles southeast of Yakima. It was a so-called "downer" animal, meaning it was unable to walk when it reached the slaughterhouse, which under USDA rules triggers automatic testing.

"While this would represent the first finding of BSE in the United States we have worked hard to ensure our response is swift and effective," she said.

Separate rounds of tests, including what Veneman termed the "gold standard" procedure for detecting BSE, done at a USDA facility in Ames, Iowa, led her to term the case "presumptive positive" for BSE.

The sample was being flown by military aircraft to the United Kingdom for a final round of testing, Veneman said. Those results are expected in three to five days, she said.

The USDA has placed the farm under quarantine.

White House Spokesman Jim Morrell says President Bush, who is at Camp David, was briefed by Veneman before the public announcement. Morrell says the White House is monitoring developments.

Mad cow disease first appeared in the United Kingdom in the mid-1980s and resulted in the slaughter of millions of cattle.

BSE spread across the European cattle industry after it first developed in the UK in the mid-1980s, but the first case in North America appeared in the Canadian province of Alberta in May. Eighteen farms were quarantined, but no additional cases were discovered.

The infectious agent takes at least six to eight years to cause symptoms in cows.

BSE is spread only by the consumption of infected feed and is not transmitted from cow to cow.

"It's an important point to make that the disease is difficult to spread," said Steve Lyle, director of Public Affairs for the California Department of Food and Agriculture.

A critic of cattle industry safety standards said the current case is likely "the tip of an invisible iceberg."

"There are more cases, no doubt about it," said John Stauber, author of Mad Cow, USA.

Accusing Veneman of underplaying the severity of Tuesday's finding, Stauber said the fact it took so long to find a case only underscores a weakness in the testing system.

"In Europe and Britain, they test virtually every beef animal for mad cow," Stauber said. "That's what we should be doing in the United States."

The National Cattlemen's Beef Association promised to support aggressive steps to eliminate any trace of BSE from the U.S. cattle population, and said the system had worked in tracking down the case in Washington state.

"Consumers should continue to eat beef with confidence," the NCBA said in a statement. "All scientific studies show that the BSE infectious agent has never been found in beef muscle meat or milk and U.S. beef remains safe to eat."

A spokesman promised a vigorous campaign to reassure the public, saying "We will be on this like white on rice, the next few days."